1. Field of the Invention
The present invention relates to a method for analyzing free fluorine in solutions containing hydrofluoric acid, and an analytical apparatus therefor. More specifically, the present invention relates to a method for analyzing free fluorine in solutions containing both hydrofluoric acid and metal ions, which enables analyzing free fluorine without being affected by the coexistent metallic ion, and an analytical apparatus therefor.
2. Description of the Related Art
Stainless steel is used in various applications for its excellent corrosion resistance and appearance. Such steel is subjected, after hot rolling, to a pickling treatment to remove scales (oxidized film) formed on its surface layer. A hydrofluoric acid-containing solution such as an aqueous mixed acid solution of nitric acid and hydrofluoric acid is widely used as a pickling solution because of the excellent descaling capability.
Component analysis of the pickling solution is important, as the pickling efficiency varies as a function of the acid consumption during pickling, and as a function of dissolution of iron ions into the pickling solution.
Consequently, the hydrofluoric acid concentration, the total acid concentration (hydrofluoric acid concentration plus nitric acid concentration) as well as the metallic ion concentration (iron ion concentration) are analyzed in consideration of the influence on the descaling capability when controlling the concentration of the above-described pickling solution for a stainless steel.
Also, in this case, it is important to control, as an indicator of hydrofluoric acid concentration, the concentration of free fluorine on which the descaling capability is dependent, that is, the concentration of effective fluorine (the sum of HF concentration plus Fxe2x88x92 concentration) which is not combined with iron or the like.
As a method for analyzing nitric acid/hydrofluoric acid for use in pickling, a method for analyzing hydrofluoric acid concentration and nitric acid concentration using an ionic electrode was reported (cf. CAMP-ISIJ, Vol. 8 (1995), p. 1980).
However, the above-mentioned method is affected by the coexistent components as their concentrations vary. In addition, the analytical accuracy is not sufficient.
Also, a method for analyzing hydrofluoric acid by titration using lanthanum nitrate or the like was reported (cf. CAMP-ISIJ, Vol. 6 (1993), p. 1311). The method is, however, dedicated to the analysis of total fluorine concentration, and cannot be applied to analyzing free fluorine which is effective for descaling.
Also, an analytical method for analyzing acids and metals according to their forms by thermometric titration and potentiometric titration was reported in the Japanese Unexamined Patent Application Publication No. 3-15749. The method is, however, inappropriate for an analytical method of total free fluorine, since it is not possible to analyze fluorine ion in solution.
An analytical method for analyzing free fluorine by the iron-acetylacetone complex fading absorptiometric method was also reported (J. P. McKaveney: Anal. Chem. Vol. 40 (1968), p. 1276).
However, in the above-described cases, it is difficult to perform analysis in a stable manner, as the absorbance changes with the passage of time, if a metallic ion such as iron ion coexists in a solution as in a pickling solution for a stainless steel, as stated later.
The present invention aims at solving problems of the above-described conventional techniques and providing a method for analyzing free fluorine in a hydrofluoric acid-containing solution which enables analyzing free fluorine without being affected by a coexisting metal and with excellent accuracy, and an analytical apparatus therefor.
Thus, the present invention is a method for analyzing free fluorine in a hydrofluoric acid-containing solution with a coexistent metallic ion wherein the concentration of free fluorine is determined based on a total fluorine concentration, a total acid concentration, a metallic ion concentration, the equilibrium constant of hydrofluoric acid, and equilibrium constants of metal fluoride complexes. It is noted that this analytical method is a method for analyzing free fluorine suitable for the case in which a hydrofluoric acid-containing solution with a coexistent metallic ion is a pickling solution for a stainless steel.
Also, the present invention provides an apparatus for analyzing free fluorine in a hydrofluoric acid-containing solution with a coexistent metallic ion comprising:
a total fluorine concentration analyzing unit for analyzing a total fluorine concentration in the above-described solution to output the analytical value;
a total acid concentration analyzing unit for analyzing a total acid concentration in the above-described solution to output the analytical value;
a metallic ion concentration analyzing unit for analyzing a metallic ion concentration in the above-described solution to output the analytical value; and
a processing unit for inputting each of the outputs from the analyzing units to calculate the concentration of the above described free fluorine based on the equilibrium constants of hydrofluoric acid and metal fluoride complexes.
In this apparatus for analyzing free fluorine, it is preferable that the above-described total fluorine concentration analyzing unit is an analytical unit employing a precipitation titration method using lanthanum nitrate, the above-described total acid concentration analyzing unit is an analytical unit for reducing a metal followed by neutralization titration, and the above-described metallic ion concentration analyzing unit is a unit for analyzing a metal-EDTA complex by absorptiometry.
More preferably, the above-described apparatus for analyzing free fluorine is further equipped with a fractionating unit to supply a sample to each of the units for analyzing total fluorine concentration, total acid concentration and metallic ion concentration.